Solid/fluid contacting



F. 1.. n. CLOETE ET AL 3,551,118

SOLID/FLUID CONTACTING '7 Sheets-Sheet l lnvenlors paw M M 77 W M,

A ttorncys Dec. 29, 1970 Filed Oct. 12, 1967 Dec 29, 1970 Filed Oct. 12,1967 L. D. CLOE'TE AL 3,551,118

SOLID/FLUID CONTACTING 7 Sheets-Sheet 2 4 Z gizenlors QMM M *Mwa AHorneys Die. 29, 1970 CLOETE ETAL 3,551,118

SOLID/FLUID CONTACTING Filod. Oct. 12, 1967 7 Sheets-Sheet 5 J L *&|--.

a I E lgvenlora A llornvys SOLID/FLUID CONTACTING Filed Oct. 12, 1967 7Sheets-Sheet 5 Dec. 29, 1970 D- cLoE'rE 3,551,118

SOLID/FLUID CONTACTING Filed Oct. 12, 1967 7 Sheets-Sheet 6 MW VMW Allorneys De,29 1 F. Lib. CLOETE ETAL SOLID/FLUID CONTACTING Filed Oct.12, 1967 7 Sheets-Sheet v nmn o 0 [III United States Patent U.S. Cl.23-621 8 Claims ABSTRACT OF THE DISCLOSURE The invention relates to aprocess for countercurrent contacting of fluids and solids in aplurality of intercommunicating solid, fluid contacting chambers whichinvolves allowing the phases to separate during the first part of eachcycle and independently transferring a portion of the settled solids.The process is useful in recovering uranium oxide, U 0 from an ore leachsolution containing sulphuric acid.

This is a continuation-in-part of our application Ser, No. 317,180,filed Oct. 18, 1963, now abandoned.

This invention relates to a process of contacting a fluid phase with aparticulate solid phase in countercurrent so as to promote mass exchangebetween the two phases, and an apparatus in which to carry out theprocess.

In certain fields of technology it has hitherto proved impossible todevise a process of solid/fluid contacting which is genuinelycountercurrent in nature, i.e. in which the motion of the two phasesthrough a contacting vessel is substantially continuous and opposite indirection. The tendency of the particulate solid to fluidize,particularly when the two phases do not differ greatly in density,upsets the independence of flow of the two phases thus precluding anyinvestigation of function in terms of theoretical plate height, and itis frequently extremely diflicult to extract each respective phase froman end of the system uncontaminated by the other.

Passing the fluid vertically downwards through a bed of the solid is nota satisfactory solution since the solid tends to pack into a compactmass giving a small plate height and creating further problems withregard to upward movement of the solid. In one such proposed system thesolid is normally stationary but is periodically moved upwards by apredetermined amount, a quantity of spent solid being removed from thetop of the column and a corresponding quantity of fresh solid beingintroduced at the bottom. Various further proposals have been putforward with the object of eliminating the undesirable disturbing effecton the main portion of the solid bed brought about by the periodicupward movement.

In accordance with the present invention, these difliculties are avoidedin a process and apparatus in which countercurrent or similar continuousfluid/ solids contacting take place, for example for ion exchange orother physical or chemical interaction, while solids are periodicallydispersed at least in part in the fluid. There are alternate periods ofdisturbance and settling of the particles such that in between timeswhen the solids are dispersed to promote interaction they settle out atleast in part to give substantial fluid/solids separation. When thesolids settle, there are created one or more substantially solidfreefluid phase volumes and one or more substantially fluid-free solid phasevolumes. Transfer of fluid and/or solids through the system is effectedby movement of fluid from one or more of these substantially fluid phasePatented Dec. 29, 1970 "ice volumes and/or movement of solids from oneor more of these substantially solid phase volumes.

More specifically the invention consists in contacting a fluid phasewith a particulate solid phase in countercurrent. A fluid feed through aseries of contactor stages containing the solid is subjected to a flowcycle, the configuration of the stages and the characteristics of theflow cycle being such that (a) during one part of each cycle the twophases are allowed to separate, (b) in another part of the cycleseparated solids from each stage is transferred into, and dispersed in,separated fluid in the next preceding stage and (c) in a further part ofthe cycle separated fluid from each stage is transferred into, anddisperses, separated solids in the next following stage.

The fluid may be liquid or gas, and the operations to which the processof the invention may be applied include bleaching, ion-exchange,crystallisation, drying, adsorption, roasting, solid-particle-catalysedgas-phase and liquidphase reactions and a variety of gasor liquid-solidchemical reactions.

In the preferred embodiment of the invention, the flow cycle applied tothe fluid phase has a wave-form including reversal of flow, and theconfiguration of the contactor stages is such that the separation of thephases occurs at the zero portion of each cycle, and the solid transferand fluid transfer from each contactor stage occurs at the negative andpositive portion respectively of each cycle. Best results are obtainedfor certain unsymmetrical waveforms and with a cycle period of fromabout 1 minute to about 1 hour.

The configuration of the stages should permit effective dispersal ofsolids in fluid during both the negative and positive portions of eachcycle, that is both when solids are introduced into the fluid-richregion of a contactor stage and when fluid is introduced into thesolids-rich region, for it is by means of such dispersal that effectivemass-exchange between the phases is brought about.

The stages should also be constructed so that at the zero-fluid-velocityportion of each flow cycle, prior to the negative portion of the cycle,as complete as possible a separation of the phases can occur by settlingof the solids. Phase separation at this point is particularly importantin order to achieve independence of flow of the two phases. Each stagemust also, of course, be provided with means communicating with adjacentstages in the series by which separated fluid can pass in the forwarddirection during the positive portion of the cycle and separated solidsin the reverse direction during the negative portion of the cycle. Thefluid velocity during the positive part of the cycle should not be sogreat as to entrain solid in the forward direction between stages.

A particular advantage of the invention is that the degree ofdispersion, the throughput and the degree of separation may be regulatedin any given case merely by controlling the wave-form, frequency andamplitude of the flow cycle applied to the fluid feed.

In one embodiment, the contactor stages are constituted by trays in avertical column. Alternatively, a series of appropriately connectedhorizontal vessels may be employed; the invention may even be put intopractice by the use of a convoluted tube, the configuration of theconvolutions of which is determined by the function which the tube is toserve.

The application of reversing cyclic flow in the form of symmetricalpulses to a liquid stream moving in counter-current to a solids streamis known from a number of earlier proposals. One such proposed systememploys a column having trays which extend across almost the whole widthof the column, having a gap for downward feed of solids; the trays areprovided with a weir over which solids must pass to reach the nextstage. During the low-fluidvelocity portions of each pulse period thesolid in each stage settles and cannot pass the weir; duringhigh-fluidvelocity portions of each pulse period the solid is fluidizedand passes over the weir in that condition. This system does nottherefore include the essential feature of the present invention, whichis that each phase passes into an adjacent stage as nearly as possiblein a monophasic condition, that is while separated from the other phase.

In another system which has been proposed previously, the liquid streamflowing through a horizontal series of contactor stages is, in eachstage, subjected to a vertical pulsation, i.e. a pulsation transverse toits direction of flow. There is in this case no necessary relationbetween the degree or kind of pulsation applied to different stages andthe pulsation is only responsible for solid phase transfer insofar asthe basically gravitational transfer of solid through the system isfacilitated by the agitation.

The invention also includes apparatus for carrying out the fluid/ solidscontacting process.

In order that the invention be more clearly understood some embodimentsthereof will now be described, by way of example, with reference to theaccompanying drawings in which:

FIG. 1 is a diagrammatic elevation of a vertical column contactor;

FIG. 2 is a detailed perspective view of a column element which may beemployed in the column of FIG. 1;

FIG. 3 is a detail of a section through a column, similar to that ofFIG. 1, which employs a further type of column element;

FIGS. 46 are diagrammatic elevations of another type of vertical columnelement illustrating the behavior of the solid held on the element atdifferent portions of an applied pulse cycle;

FIGS. 7 and 8 show the effect on the fluid flow rate and solid flow raterespectively of a single flow cycle;

FIG. 9 shows diagrammatically a contactor comprising a number ofinterconnected horizontal stages;

FIGS. 10-12 illustrate the behavior of the solid held on one of thestages shown in FIG. 9 at difiFerent portions of the flow cycle;

FIG. 13 is a diagrammatic elevation of a further type of vertical columncontactor;

FIG. 14 is a detail of an alternative arrangement to that shown in FIG.13;

FIG. 15 is a more general view of the arrangement of FIG. 14;

FIG. 16 is a flow diagram of an ion-exchange process employing theinvention;

FIGS. 17 and 18 illustrate a suitable means for generating the flowcycle; and

FIG. 19 illustrates a second means for reversing the fluid feed.

FIG. 1 of the drawing shows a column 1 having an enlarged upper portion2 from which fluid may be withdrawn by a conduit 3 and into which solidsmay be introduced by a conduit 4. The lower part of the column 1 is incommunication with the balancing leg 5 of a flow cycle generatorindicated schematically at 6, and the fluid feed, upon which thevariations produced by the generator 6 are imposed, is through a conduit7 positioned a little above the junction of the column 1 and thebalancing leg 5.

The part of the column 1 between this junction and the enlarged upperportion 2 contains a number (only four shown) of contactor elements 8whose construction is shown in detail in FIG. 2 and is hereinafter morefully described. The portion 11 of the column 1 which lies below thejunction of the column with the pulse leg 5 forms the pathway forremoval of solids from the system.

In operation a constant fluid feed is applied between conduits 7 and 3and a constant solids input is drawn through the conduit 4, the solidsflowing down the column under the combined influence of gravity and thenegative portions of the flow cycles induced by the generator 6.

41 The solids are eventually discharged from the system at 11.

An element 8 is shown in detailed perspective in FIG. 2, and consists ofa conical dish 12 which occupies the whole cross-section of a column andis by-passed by a circular conduit 13 one end of which takes ofl fromthe lower part of the inside of the dish (which is mounted in the columnwith its apex lowermost). As can be seen from FIG. 1, the take off ofthe upper end of conduit 13 is below the rim of dish 12 so that it isbelow the level of solids which settle in the dish at the zero part ofthe cycle. The lower position of the by-pass terminates in a shortvertical portion below the apex of the dish. During operation solidsfall into the bottom of each dish at lowfluid-velocity portions of eachcycle and cover the mouth of conduit 13; at negative portions of thecycle solids are sucked from the bottom of the dish 12 through a conduit13 and drop into the fluid supernatant in the next dish; conversely, atpositive portions of the cycle fluid is forced upwards through theconduit 13 into the solids in the bottom of the dish 12. These movementscause the solids to disperse from the dish 12 into the fluid above it.Thus at both positive and negative portions of the cycle there is phasedispersion.

A section of large-diameter column containing an alternative type ofcolumn element is shown in FIG. 3. Each element is mounted on animpermeable tray 17 extending across the column and consists of a highchimney 14 which passes through the tray and is surmounted by a domedcap 15 having an extended impermeable skirt 16.

In operation, the depth of solids on each tray is greater than thedistance of the bottom of the skirt 16 above the tray surface: fluidpassing up through the chimneys 14 when the cycle is positive is thusforced through the solids settled on the tray 17; when the cycle isnegative, settled solids are sucked up the gap between the skirt 16 andthe chimney 14 and dropped into the liquid overlying the solids on thenext tray down the column.

A section of a large-diameter column containing a further type of columnelement is shown in FIGS. 4, 5 and 6 each of which illustrates thebehavior of the solids at a different portion of the cycle. Each elementcomprises an inverted conical plate 20 which is attached to the wall 22of the column and which has an aperture 24 at its apex to allow passageof the phases from one stage to the next. Beneath the aperture 24 thereis located a small V-shaped baffle 26 which serves to prevent solidstransferring downwards except during the negative portion of the cycleand also, as will hereinafter become apparent, to distribute thedescending solids across the cross-section of the column.

FIG. 4 shows the behavior of solids 26 held on the plate 20 duringpositive portions of the applied cycle, when the fluid passes upwardsfrom the next preceding stage through the aperture 24 and the solids 26held on the plate 20 which is thus dispersed an tends towards thefluidized state. Mass transfer between the two phases occurs during thisportion of the cycle.

As the flow approaches that portion of its cycle where the fluid flowrate is at a minimum, the gravitational forces acting on the solidparticles overcome the small force of ascending fluid in the oppositedirection and the particles begin to settle as indicated in FIG. 6,which, compared with FIG. 5, shows a reduced solids volume. During thisportion of the cycle the phases separate until by the time the fluidflow is reversed substantially complete separation of the fluid from thesolids held on the plate has occurred.

When the fluid flow is reversed, the solid particles are sucked from theplate 20 through the aperture 24, as indicated in FIG. 6, and aredistributed by the baffle 26 evenly on to the next lower plate and aredispersed in the fluid supernatant thereon. The cycle is completed asforward flow of fluid is resumed and reproduces the condition shown inFIG. 4.

-It will be apparent from the above that the degree of separation of thephases and the degree of their dispersion at the different parts of thefluid cycle, and hence the degree of contamination of the end productsand the quantity of mass transfer achieved in any given case, depends toa very large extent on the wave-form of the app-lied flow cycle, itsfrequncy and its amplitude. A wave-form suitable for liquid/ solidstransfer in a vertical column is shown in FIG. 7 which is a graph offluid velocity (in ft./ min.) plotted against time (min.).

The wave-form shown therein is asymmetrical and as indicated on thegraph consists of four distinct portions. During ortion 1 the fluidvelocity is positive (i.e. the fluid is ascending the column anddispersing the solid phase, as described above with reference to FIG. 4)and is constant. It will be seen that the duration of portion 1 isapproximately half the time of the complete cycle. A high degree ofphase mixing and hence mass transfer thus occurs during this stage. Themaximum flow rate is limited by the terminal falling velocity of thesolid particles in the fluid which must not be exceeded if entrainmentis to be avoided.

In portion 2, the fluid velocity rapidly decreases to zero and ismaintained thereat for the relatively short time necessary to achievesubstantially complete phase separation, as described above withreference to FIG. 6. The minimum time for this part of the cycle is veryshort and may range from less than 1 second for gas-solid systems to upto seconds for ion-exchange resins in water.

In portion 3, the fluid velocity rapidly reaches a maximum from which itequally rapidly returns to zero. During this portion of the pulse cycle,solids are transferred from one contactor stage to the next, asillustrated in FIG. 8 which is a graph of velocity of solids flowplotted against time for a flow cycle of the wave-form shown in FIG. 7.It will be appreciated that the amount of solids transferred during thisportion can be controlled positively, and in all cases should be lessthan about 50% in order to avoid any tendency of solid particles toby-pass a complete stage. In most cases, from about 1 to 30% of solidsis transferred and is usually accompanied by about the same volume offluid.

In the last portion, portion 4, the fluid which has been entrainedduring the solids transfer is returned by normal fluid upflow whichfairly quickly reaches the maximum and constant value which it has inportion 1.

The average velocity of the fluid over the complete cycle is indicatedin FIG. 7 by the dotted line marked u and is positive, which is to saythat over each cycle there is a net flow of fluid up the column.Similarly the average velocity of the solid flow is indicated at u inFIG. 8 and is negative, which is to say that over each cycle there is anet flow of solids down the column. From the point of view of itsperformance, therefore, it will be seen that the method of the inventioncan be made to approximate to a continuous countercurrent process.

The arrangement shown in FIG. 9 differs from those previously describedin that the motion of the counterflowing phases is through a horizontalseries of contacting stages; since the parts indicated by the referencenumerals 2, 3, 4, 5, 6, 7 and 11 do not differ materially inconstruction or function from the parts similarly designated in FIG. 1these will not be further described. The arrangement consists basicallyof a series of vessels 18 interconnected by conduits 19, of diameterless than half that of the vessels 18, which join the bottom part ofeach vessel 18 with the top part of the vessel 18 preceding it in theseries; the behavior of the solid phase in each of the stages shown inFIG. 9 at portions of the flow cycle corresponding to positive fluidvelocity, zero fluid velocity and negative fluid velocity is illustratedin FIGS. 10-12 respectively. It will be seen form these drawings thatthe behavior of the solid phase is strictly analogous to the behavior ofthe solid phase in the elements shown in FIGS. 4-6 and described above.

During operation, it is desirable to maintain a constant hold-up ofsolid phase on each contactor stage in order to achieve a reasonabledegree of efficiency of mass transfer in each stage and thus to keep thenumber of stages required for a given degree of transfer withinreasonable limits. A column having means for ensuring a constant hold-upis shown in FIG. 13. Therein each contactor stage is shown as comprisinga plate 30 which extends part way across the column and which has alarge number of perforations 32 of a diameter considerably larger thanthe size of the solid particles to be dealt with by the column.

Each stage is separated from its adjacent stages by an impermeableconical baflle 38 located beneath the perforated plate 30, and thecolumn space between each plate 30 and the baflle 38 of the adjacentfollowing stage is divided into two compartments 40 and 42 by animpermeable partition 44 which extends from the column wall on one sideto the inner edge of the plate 30. A partition 46 divides the spacebetween the plate 30 and the baffle 38 into two compartments 48 and 50which communicate with each other through a small space 52 between thelower edge of the partition 46 and the apex of the balfle 38. Each stagecommunicates with its adjacent stages for passage of the phases througha conduit 54 external to the column which connects the space 42 of onestage to the space 40 of the next preceding stage.

In operation of the column a bed of solids will be held on the plate 30.At substantially zero fluid flow a proportion of the solids bed willsettle through the perforations 32 and into the space 48. During reverseflow of fluid, solid will be drawn from the space 48' into the space 50and from there into the adjoining space 42, and in due course will betransferred by the conduit 54 to the space 40 and the plate 30 of thenext preceding stage.

Upon forward flow of fluid, such solids as have been carried, during thepreceding negative flow period, into the spaces 48, 50 and 42 and partof the conduit 54 will be carried back through the plate 30 to becomeagain part of the solids bed thereon. It will be apparent, therefore,that solids associated with each plate will perform a to-and-fro motionthrough the plate perforations and within the spaces 48, 50 and 42, withsome solids spilling down through the conduit 54 to the stage belowduring each negative flow period; but no such spillage 'will take placeuntil a particular solids hold-up has been achieved in that stage, whichhold-up is thereafter maintained substantially constant.

Referring now to FIGS. 14 and 15, another way of providing for hold-upof the solid phase in each stage is to support the bed on a pair ofspaced plates 101, 102 one close below the other and each perforated,with the perforations 103, 104 in the two plates mutually offset. Therewill then be a tendency for each perforation in the upper plate to beblocked by a pile of solids 105 resting on the lower plate as shown inFIG. 14. During each negative flow period, however, there will be carrythrough of solids by the fluid into the stage below. In order to controlthe amount of solids left behind on the plates when this carry throughby the fluid occurs, it is advantageous to provide a bypass conduit 106(FIG. 15) around each plate pair which incorporates a valve 107operated, either manually or automatically, in accordance with theobserved or instrument-sensed hold-up on the plates, as by means of asolids level indicator 108, whereby a regulated amount of the fluidflowing from stage to stage during the negative flow period can bebypassed without going through the plates.

The invention Will now be further illustrated by the followingdescription of a process of recovering uranium oxide U 0 from a typicalore leach solution containing free sulphuric acid to a pH of 1.8 and 1.0gram of U 0 per liter. A flow-sheet of the apparatus used is shown inFIG. 16. The process is designed to recover 400 kilograms of U in each24 hours of operation. The solids phase is a strongly basic anionexchange resin in the size range to mesh, which after the adsorption iscompleted is eluted by the method of the invention with a 1.0 molaraqueous solution of sodium chloride to give a solution containing 10.0grams of U 0 per liter from which pure U 0 is subsequently recovered byprecipitation with ammonia.

The mass transfer is effected in two columns C C (see FIG. 16) which areof the type described above with reference to FIG. 13 i.e. equipped withmeans for maintaining a constant hold up of solids in each contactorstage. The details of the columns C C and other items of equipment aregiven at the end of this description.

In order to give the entire operating conditions of the process, a shortdescription of it will now follow.

Feed solution is pumped at a rate of 16.75 m. /hr. from storage by acentrifugal pump P via a flow controller F to stage 2 of the column CThe flow cycle is applied to the liquid in the column by pumps P and Pthrough the tanks T or T and its duration is 7 minutes made up asfollows: liquid feed at an instantaneous flow rate of 23.5 m. /hr. for 5minutes; one minute when no flow of liquid takes place and the solidssettle on the stages; and one minute when the two way valves V V aroundpump P reverse the liquid flow through the column at an instantaneousliquid flow of 5 m. /hr. transferring settled solid at an instantaneousflow rate of 2.46 m. /hr. Thus during the minute of fluid flow down thecolumn, 0.041 cubic meter of solids are transferred from one stage tothe next preceding stage, and from stage 1 to the resin collecting tankT 3 or T For each complete cycle, therefore, the average liquid flowrate is 16.75 m. /hr. and the average solids flow rate is 0.35 m. hr.

Included within the 5 minutes of feed flow is one minute during whichthe pump P returns to the column the 0.084 cubic meter of liquid whichit withdrew from the column during the negative portion of the cycle.After this minute, the pump P is recirculating water in the wash waterfeed tank for the next five minutes. Pump P feeds wash watercontinuously to the column at a rate of 0.05 m. /hr. which issufficiently small not to interfere with the operations described above.

Similarly for the elution column C eluent liquor containing 1 molarsodium chloride is pumped from storage by pump P through flow controllerF to stage 2 of column C The cycle period for this column is 3 minutesmade up of 1 minute of settling with no flow, 1 minute of reverse flowat a liquid flow of 2.10 cubic meters per hour and 1 minute of eluentfeed at 3.45 cubic meters per hour. During the minute of eluent feedpump P returns the 0.035 cubic meter to the column which it withdrewduring reverse flow. Hence the column stages are designed for a maximuminstantaneous liquid flow rate of 5.55 cubic meters per hour. Theaverage eluent feed rate is thus 1.15 m. /hr. and the average resin flowrate is 0.35 m. /hr. The resin leaving the column is washed by acontinuous flow rate of water of 0.05 cubic meter per hour from pump Pas in column C The resin product from column C collects in one of thecollecting tanks T or T Only one of these is connected to stage 1 at anyone time, the other being isolated from the column by valve V or V Whenone collecting tank is full, every three hours approximately, it isisolated from the column and the other tank then connected to thecolumn. The full collecting tank is then pressurized with mains waterand the resin forced out through the pipe at the base to the appropriatefeed tank to the eluting column C i.e. either tank T or T In a similarmanner eluted resin is transferred from tanks T and T to feed tanks Tand T Resin is fed to the columns from the feed tanks T T T and T byforcing settled resin out from the base of the appropriate tank Lil withmains water at a rate controlled by flow controllers F and F to column Cand C respectively.

The details of the columns C C are as follows:

Diam- Stage Column eter, m. spacing, m.

The details of the pumps are as follows:

An important advantage over conventional techniques of the abovedescribed process for recovering uranium oxide is that the necessity forfiltering the ore leach solution to remove the small quantities ofsolids usually entrained therewith is avoided as the action of thecontactor column is not seriously affected by the presence of smallquantities of solid in the liquid phase.

Amongst many other applications of the invention may be mentioned thepurification of water by removal of cations such as calcium, sodium, andmagnesium On a cation ion-exchange resin using two contactors, one ofwhich absorbs the ions on to the resin, producing water free of metalliccations, while the other contactor regenerates the used resin from thefirst contactor with a strong acid solution. The objectionable anions,such as sulphate and chloride, in the water can subsequently be removedby a similar process using an anion-exchange resin which is regeneratedwith strong alkali solution.

A similar combination of two contactors can also be used to exchangehydrogen ions for sodium ions, producing a fatty acid from a fatty acidsalt in one unit and regenerating the resin to the hydrogen form withstrong acid in the second unit. Other useful ion-exchange processeswhich may be effected include the recovery of chromium and similarvaluable metals from plating waste solutions; recovery of gold fromleach solutins; the ion exchange or charcoal decolorizatiou andpurification of sugar solutions; and dye solution deionization. Yetfurther applications are in the leaching of coarse ground oxidizedcopper ore with sulfuric acid, and roasting of pyrites minerals toproduce sulfur dioxide.

In general, the maximum instantaneous value of the fluid flow rateduring any part of the cycle is limited by the need to prevent thecarry-over of some solid particles by the fluid flowing from one stageto the next. Some guide to this fluid velocity may be easily obtained insmall-scale experiments or from data published by manufacturers of thesolids handled. For example, for ion-exchange resin in the standardindustrial size range of l4 to +52 mesh the maximum superficial linearwater velocity is of the order of 15 centimeters per minute. From his,the stage diameter required to handle any given quantity of fluid can becalculated. The pressure drop over the equipment can be estimated fromstandard data and hence the pump duty specified.

Unless the waveform of the flow cycle is very unusual, the limitingfactor in design of equipment is the flow rate of fluid, and once thishas been used to size the stages, the solids flow rate used isdetermined by the proportions and waveform of the flow cycle. Pressuredrop during reverse flow of solids can again be estimated for designpurposes from published data.

It must be borne in mind that the overall or average flow rates of fluidor solid depend both on the maximum instantaneous flow rate and theproportion of the cycle period during which it is obtained. Thus for anygiven operating plant the ratio of the average flow rates of solid tofluid may be altered by simple adjustment of the cycle programcontroller. A contactor may operate anywhere within the range from zeroaverage fluid flow with maximum average solid flow to zero average solidflow with maximum average fluid flow. This is an extremely impor tantadvantage of the invention.

The determination of the number of stages to be used for a givenseparation can be carried out by using the cnventional McCabe-Thielemethod for a continuous countercurrent stagewise process. Mass transferruns were carried out in a small scale contactor in which a syntheticcation ion-exchange resin was passed countercurrent to an aqueoussolution through the contactor. On the caesium/ hydrogen chloride systemmeasurements showed that the stage efficiency was between 40% and 85% ofthe theoretical value for an ideal stage based on a McCabe-Thielediagram.

As has been indicated above, the solids may sometimes be fed to thecontactors in the form of a slurry in the case of liquid/solid systems.The feed may be drawn from a slurry feed tank, which is provided with anagitator to keep the slurry homogeneous and maintain a constant solidsinput rate, by a variable speed positive displacement rotary pump.Alternatively, a variable speed positive displacement rotary pump mayimpel a liquid into the top part of a slurry settling tank thus forcinga setfled-slurry feed from the bottom of the tank.

Similarly, solids product can be removed from the system by being fedinto a settling tank from the bottom of which the settled slurry isperiodically forced by hydraulic pressure. The tank may simply be anextension of a contacting column, fluid being injected and the settledslurry withdrawn, by variable speed positive displacement rotary pumpsthe relative speeds of which can be adjusted so that no fluid flows intoor out of the actual contacting columns. With this type of pump thesolid slurry output can be made independent of variation brought aboutby the variations of flow in the column.

An example of a suitable cycle generator which may be used forcontrolling the fluid cycle will now be described with reference toFIGS. 17 and 18 of the drawings which show a cycle generator in sectionand in plan respectively.

A pipe 60 is connected at one end 62 to a source of compressed air (notshown) and at its other end 64 to the balancing leg. The pipe 60 has alimb 66 which ter minates in a nozzle 68 from which a stream of thecompressed air escapes. Facing the nozzle 68 is the surface of a cam 70,rotatably mounted in bearings (not shown), which is so close to thenozzle 68 as to impede the escape of air from that nozzle. Both the pipe60 and the cam 70 are mounted on a base-plate 72 with respect to which,'however, the pipe 60 is movable by adjustment means '74 by which themean distance of the nozzle 68 from the cam 70 can be varied.

- In operation, the air pressure at 62 is dissipated at nozzle 68 andfeed input 64. The amount of air permitted to escape through nozzle 68depends on the closeness to the nozzle of the surface of cam 70;rotation of the cam alters this amount, and this alteration isreproduced as a cyclic variation in the feed output 64.

The frequency and waveform of this variation are determined respectivelyby the speed and shape of the cam 70; its amplitude may be increased byoperating the adjusting means 74 to bring the cam 70 closer to thenozzle 68.

The cycle generator just described is primarily intended for use incontrolling the flow of a liquid supply, but it can also be employed incontrolling the flow of a gaseous supply, especially where thesubsequent path of the gas stream is short and undue distortion of theimposed waveform by reason of the elasticity of the gas will not occur.It may of course, be necessary to take precautions to ensure that 10 apoisonous or otherwise dangerous gas is not allowed to escape from thenozzle of the cycle generator into an environment where its presence isundesirable.

Referring now to FIG. 19, this illustrates an embodiment of a gas/ solidcontacting system in accordance with the invention which systemincorporates an alternative arrangement for controlling the gaseoussupply.

The actual series of contacting stages is not shown in the figure sincethe contacting stages can be similar in construction to any of thosealready described.

In the figures, a rotary or centrifugal blower draws a stream of gasfrom a gas holder 82 via valve 84 and expels it, via valve 86, into theseparator 88 of the series of contactor stages; the gas holder 82 ismeanwhile supplied from a source 88 via valve 90. Provision is made forautomatic reversal of the direction of gas flow in the flowpath betweenthe gas holder 82 and the separator 88; this reversal is accomplished byopening valves 92 and 94 simultaneously with closure of valves 84, 86and 90. The blower 80 then draws gas from the separator 88 and deliversit into the holder 82 until this latter fills to a predetermined level,whereupon the five valves are triggered back to their original settingsto restore the initial direction of flow. The periodic reversals of gasflow into the separator appear as regular flow reversals in the gas feedin a connection 96 between the separator 88 and the actual contactingsystem. Solid is withdrawn from the system by a rotating star valve 98in the base of the separator.

It will be apparent that the invention has wide application in thechemical processing industries. In addition to the advantages alreadymentioned, the invention enables many processes to be carried out atmuch reduced operating and capital costs. In some cases it may bepossible to adapt plant operating according to existing techniques towork by the method of the invention, and this will naturally lead to asaving in capital costs. In yet other cases, the invention may makefeasible processes which hitherto have been but theoretical curiosities.

What is claimed is:

1. A process for countercurrent contacting of fluids and solids whereinion exchange or other physical or chemical interaction occurs while thesolids are dispersed at least in part in the fluid comprising:

flowing a fluid phase countercurrent to a solid phase through a seriesof contactor stages containing the solid, in a plurality of cycles,

allowing said two phases to separate during a first part of each cycle,

then transferring a portion of the separated solid from each contactorstage substantially independently of the separated fluid in that stageto the separated fluid in the next preceding stage, while said liquidand solid are separated, during a second part of each cycle, and

transferring a portion of the separated fluid substantially free ofentrained solids from each contactor stage into the separated solid inthe next following stage, and thereby redispersing the solid in eachstage in the fluid, prior to allowing the fluid and solid in each stageto separate in the first part of the next cycle.

2. A process for countercurrent contacting of fluids and solids in aplurality of intercommunicating solid-fluid contacting chambers whichcomprises:

introducing solid material into a first fluid-solid contacting chamber,introducing fluid into a subsequent fluid-solid contacting chamber, forcountercurrent flow towards said first chamber,

flowing said fluid from said subsequent chamber to said first chamber,

intermittently interrupting flow of said fluid through said chambers toallow solids in each chamber to settle under the influence of gravity,

intermittently reversing the flow of. said fluid and transferringsettled solids, substantially free of entrained 1 1 1 2 fluid, from eachfluid-solid contacting chamber to the 8. A process as set forth in claim2 in which the solids next subsequent contacting chamber, andtransferred to said next adjacent chamber are introduced resuming saidcountercurrent flow of said fluid, the fluid into an upper portion ofsaid next adjacent chamber to entering each chamber through a lowerportion theremove downwardly through the separated liquid therein. of toresuspend said solid material for eflicient fluid- Solid Contact 5References Cited 3. A process as set forth in claim 2 in which saidfluid UNITED STATES PATENTS flow has an assymmetrical waveform.

4. A process as set forth in claim 2 in which the veloc- 2 et a1 ity offluid, during transfer of fluid in said countercurrent ury 10 3,056,74310/1962 ElChhOI'H et al. 210-189 flow, uses to a preferred value and ismaintained at send 2 932 552 4/1960 Weiss et a1 23 338 preferred valuefor a time.

5. A process as set forth in claim 2 in which the dura f 210 33 tion ofa flow cycle of forward, interrupted and reverse e a flow is greaterthan 30 seconds. 1

6. A process as set forth in claim 2 in which, for each 5 LELANDSEBASTIAN Primary Exammer cycle, the quantity of solid transferred fromeach cham- US. CL bt t d'hb'lth507fth gf gg fig 522? 2; am er ess an 023 1, 177, 299, 310, 337, 338; 75-1, 101; 127--46;

7. A process as set forth in claim 2 in which the level 20 of solidsheld up in each chamber is maintained substan tially constant during thesame part of each cycle.

